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[ research report ]

Neuromuscular electrical stimulation (NMES) is widely used in reha­bilitation to assist with

strength recovery,47 muscle re­education,25 prevention of atro­phy,3 and reduction of functionallimitation.50 NMES has also been inves-tigated for its potential role in helping to improve muscle performance in both healthy individuals and athletes.21,31,39

In 1977, at a conference in Canada, the Russian scientist Yakov Kots reported on the use of a medium-frequency alternat-ing current of 2500 Hz in Olympic ath-letes from the former Soviet Union and claimed that NMES could produce force gains of up to 40% in elite athletes com-pared to the use of exercises alone. These impressive strength gains were purport-edly achieved by using NMES to elicit muscle contractions that were 10% to 30% greater than those achieved with a

TT STUDY DESIGN: Repeated-measures, within-subject crossover trial.

TT OBJECTIVES: The primary objective was to as-sess the effect of the burst-duty-cycle parameters of medium-frequency alternating current on the maximum electrically induced torque of the quad-riceps femoris. The secondary objectives were to evaluate the amount of discomfort tolerated and the maximum current amplitude delivered for each electrical-stimulation condition.

TT BACKGROUND: Neuromuscular electrical stimulation used for muscle strengthening can improve functional performance. However, the electrical-stimulation parameters to achieve opti-mal outcomes are still unknown. Previous studies have demonstrated that the characteristics of the burst duty cycle of medium-frequency alternating current influence torque-generation levels and perception of sensory discomfort.

TT METHODS: The maximum electrically induced torque was assessed with a medium-frequency alternating current, with a carrier frequency of 2500 Hz and a modulated frequency of 50 Hz. The current amplitude was gradually increased to the point of the participant’s maximum tolerance

level. The testing sequence for the 3 burst duty cycles (20%, 35%, and 50%) was performed in a randomized order.

TT RESULTS: Electrical stimulation using a 20% burst duty cycle produced an electrically induced torque greater than the 35% (P = .01) and 50% (P<.01) burst duty cycles, with no difference between the 35% and 50% burst duty cycles (P = .46). There was no difference in the amount of sensory discomfort produced by the 3 durations of burst duty cycles (P = .34). There was also no dif-ference between the 3 conditions for the maximum current amplitude tolerated (P = .62).

TT CONCLUSION: The burst duty cycle of 20% of medium-frequency alternating current, compared to burst duty cycles of 35% and 50%, produced higher peak torque of the quadriceps femoris in professional soccer players. There was no difference in discomfort produced and current amplitude tolerated between the different burst-duty-cycle conditions. J Orthop Sports Phys Ther 2013;43(12):920-926. Epub 30 October 2013. doi:10.2519/jospt.2013.4656

TT KEY WORDS: current, electrical stimulation, tolerance

1Master’s and Doctoral Programs in Physical Therapy, Universidade Cidade de São Paulo, São Paulo, Brazil. This study was approved by the Ethics Review Board of the Universidade Cidade de São Paulo (protocol number PP 13323922) and was supported by the Universidade Cidade de São Paulo. The authors certify that they have no affiliations with or financial involvement in any organization or entity with a direct financial interest in the subject matter or materials discussed in the article. Address correspondence to Dr Richard E. Liebano, Master’s and Doctoral Programs in Physical Therapy, Universidade Cidade de São Paulo, Rua Cesário Galeno, 448/475, Tatuapé, CEP 03071-000, São Paulo, Brazil. E-mail: liebano@gmail.com T Copyright ©2013 Journal of Orthopaedic & Sports Physical Therapy®

RICHARD ELOIN LIEBANO, PT, PhD1 • SILAS WASZCZUK, JR., PT, MSc1 • JULIANA BARBOSA CORRÊA, PT1

The Effect of Burst­Duty­Cycle Parameters of Medium­Frequency Alternating Current on Maximum Electrically Induced Torque of the Quadriceps Femoris, Discomfort, and Tolerated Current Amplitude

in Professional Soccer Players

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journal of orthopaedic & sports physical therapy | volume 43 | number 12 | december 2013 | 921

maximum voluntary isometric contrac-tion (MVIC).56

Although there is still a lack of strong evidence on the advantages of NMES compared to voluntary exercise,6,21 train-ing programs for athletes that include NMES have been investigated.2,11,19,31,39 In soccer players, a 5-week training program using NMES produced increases in knee extension torque and ball speed with kicking, showing an improvement in the specific tasks of soccer.7 However, more investigations are needed to examine the optimal stimulation parameters appro-priate for NMES strengthening protocols in athletes.

Several authors have demonstrated the influence of stimulation parameters, such as electrode positioning,20 current frequency,38,44,53-55 current amplitude (in-tensity),8,22,44 stimulus pulse duration,22,23

and burst duty cycle of medium-frequen-cy alternating current,33,37,55 on electri-cally induced torque. Burst duty cycle is an important parameter of NMES, as it can directly affect torque production and sensory discomfort in healthy, un-trained individuals.33,55 The burst duty cycle of medium-frequency alternat-ing current, expressed as a percentage, can be defined as the ratio of the burst duration to the total time of the cycle (burst length and interburst interval) (FIGURE 1). Kots used a burst duty cycle of 50% to produce force gains in elite athletes56; however, this burst duty cycle appears to have been an arbitrary choice, because lower burst duty cycles were not tested in his preliminary studies. Recent research has demonstrated that lower burst duty cycles (10%-20%) are optimal for generation of maximum electrically

induced torque in the biceps brachii,4 finger flexors,35 and wrist extensors55 and produce less discomfort.35,55 Despite the common use of NMES in clinical practice to increase the strength of knee extensor musculature,6,29,38,45-47,50 only 2 studies in the literature have assessed the effect of burst-duty-cycle duration on torque pro-duction of the quadriceps in healthy, un-trained individuals.33,37 The first study33 compared 5 burst-duty-cycle durations (10%, 30%, 50%, 70%, and 90%) and found that a burst duty cycle of 10% elic-ited the strongest muscle contraction. The second study37 evaluated the effects of burst duty cycles of 10% and 90% of a medium-frequency alternating current (2500 Hz) and determined that the burst duty cycle of 10% produced stronger quadriceps muscle contractions.

There is a positive association be-tween current intensity, torque genera-tion, and isometric force gains resulting from NMES.21,47 As the current intensity of NMES increases, a larger number of motor units are activated, but discomfort also increases.1,8,24 Thus, discomfort dur-ing electrically induced torque is consid-ered a limiting factor in the effective use of NMES.18 Regular physical activity is associated with specific changes in pain perception, and psychological and biolog-ical factors may be responsible for these changes.48 Competitive athletes, who are motivated to adapt to certain levels of discomfort related to physical train-ing, demonstrate higher pain thresholds and pain tolerance when compared to nonathletes.26,27,32,48 Accordingly, a study performed with an elite weightlifter sug-gested that athletes were able to tolerate higher levels of current intensity during NMES than untrained individuals.16

A regular exercise training program can also cause metabolic and neuromus-cular adaptations in muscle tissues.41 Muscle and connective tissue can adapt to physical training by increasing tissue mass, reducing body fat, and increas-ing maximum force production.12 These factors could also lead to different out-comes when using NMES to improve

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FIGURE 1. (A) Burst duty cycle of 20% (burst duration is 4 milliseconds and interburst interval is 16 milliseconds). (B) Burst duty cycle of 35% (burst duration is 7 milliseconds and interburst interval is 13 milliseconds). (C) Burst duty cycle of 50% (burst duration is 10 milliseconds and interburst interval is 10 milliseconds). In examples A, B, and C, the carrier frequency was 2500 Hz, the phase duration was 200 microseconds, and the burst frequency was 50 Hz.

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922  |  december 2013  |  volume 43  |  number 12  |  journal of orthopaedic & sports physical therapy

[ research report ]muscle strength in athletes compared to nonathletes.

Considering the potential benefits of NMES for improving athletes’ perfor-mance and the limited number of stud-ies in this population, the primary aim of this study was to assess the effect of burst-duty-cycle parameters of medium-frequency alternating current on the maximum electrically induced torque of the quadriceps femoris. The secondary objectives were to evaluate the amount of perceived discomfort and the current am-plitude delivered in professional soccer players for each of the burst-duty-cycle conditions. We hypothesized that lower burst duty cycles would generate higher torques, while causing less discomfort, than higher burst duty cycles.

METHODS

Subjects

Thirty male professional soccer players (mean age, 18.8 years [range, 18-23 years]; body mass,

74.3 kg [range, 64.5-86.5 kg]; height, 1.78 m [range, 1.68-1.90 m]) from the Grêmio Barueri team, who had at least 3 years of playing experience and a weekly training time that exceeded 24 hours, participated in the study. All volunteers had undergone the same physical train-ing for the 8 months prior to data collec-tion. Using previously published data, sample size was calculated based on detecting a difference of at least 7.1% in torque generated with the knee extensors between conditions and a standard devia-tion of 12.6%.28 At a significance level of .05 and a power of 80%, the required sample size was 25 participants (Minitab Version 15; Minitab Inc, State College, PA). Therefore, allowing for attrition, 30 participants were recruited.

Potential subjects were excluded if they had a history of surgeries in the dominant lower limb, recent muscu-lar lesions in the quadriceps femoris (6 months prior to the study), or contra-indications to NMES. The participants signed a written informed consent form

prior to participation in the study, which was approved by the Research Ethics Committee of the Universidade Cidade de São Paulo.

Maximum Voluntary Isometric ContractionTorque production was assessed using an isokinetic dynamometer (CYBEX Norm 6000; Computer Sports Medicine, Inc, Stoughton, MA), which was calibrated prior to the study. The software used for data acquisition was the HUMAC NORM Version 9.3.2 (Computer Sports Medicine, Inc). All subjects warmed up on an unloaded, ergometric bicycle at a moderate speed for 5 minutes, followed by stretching (3 repetitions of 30 sec-onds each) the quadriceps muscle and hamstrings.15

The preferred lower extremity to kick a ball was designated as the dominant lower extremity. The volunteers were seated in the isokinetic dynamometer chair, their hips stabilized at 85° of flex-ion with a belt and their nondominant limb immobilized with a double pad lo-cated at the distal third of the tibia, 3 cm proximal to the ankle joint. Straps were used to stabilize the trunk to avoid pos-sible muscle compensation.

The dynamometer was set to an an-gular velocity of 0°/s (isometric mode) and the dominant limb was positioned at 60° of knee flexion, as measured by the dynamometer’s goniometer. The pad of the dynamometer resistance arm was po-sitioned 3 cm proximal to the ankle joint, allowing for full ankle dorsiflexion. The rotation axis of the dynamometer was aligned with the rotation axis of the tested knee. The upper limbs rested on the lat-eral support of the dynamometer chair.2,30

The peak knee extension torque that could be generated by the participant was determined using an MVIC performed at 60° of knee flexion against the isokinetic dynamometer resistance arm. Three tri-als were performed, each trial lasting 9 seconds, and the peak torque was record-ed for each trial.28,30 To minimize fatigue, a 5-minute interval was provided be-

tween each repetition.43,45 Verbal encour-agement was given by the investigator for each trial. The average of the peak torque values obtained for the 3 trials was calcu-lated and used for subsequent analysis.

Maximum Electrically Induced TorqueTwo carbon-filled, silicone rubber elec-trodes measuring 10 × 5 cm were ap-plied over the motor points of the rectus femoris and the vastus medialis46,49 with carboxyvinyl polymer gel (Carbogel In-dústria Comércio, São Paulo, Brazil) and secured with self-adhesive tape and an elastic bandage. The location of the mo-tor points was determined by a universal pulse generator (NeMESys 941; Quark Produtos Médicos, Piracicaba, Brazil), which had a current amplitude range of 0 to 70 mA. After placing the dispersive electrode (with foam pad) over the middle third of the anterior surface of the tested quadriceps, the stimulation-pen elec-trode was used to search for the motor points.49 The parameters used to deter-mine the motor points were a 200-mil-lisecond pulse duration, 500-millisecond interpulse duration, and monophasic rectangular pulsed current. The point over which the stronger excitation was obtained with the lowest amount of cur-rent applied was considered the motor point.49 Application of the electrode over the motor point of the muscle was chosen to standardize electrode positioning and to reduce the required current intensity.46

NMES was performed using the Endophasys-R (KLD Biosistemas Eq-uipamentos Eletrônicos Ltda, Amparo, Brazil) within the following current pa-rameters: 2500-Hz carrier frequency, 50-Hz modulated frequency, 200-mi-crosecond phase duration of 9-sec-ond stimulation time (with a 3-second ramp-up),29,40,42,45,46,49 and 5-minute rest between repetitions.43,45 A 3-second ramp-up was implemented to prevent a sudden and intense muscle contraction, and 5 minutes between repetitions was used to minimize fatigue.

The maximum current amplitude (0-150 mA) used for stimulation for

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journal of orthopaedic & sports physical therapy | volume 43 | number 12 | december 2013 | 923

each burst-duty-cycle condition was de-termined by the tolerance of each par-ticipant. During the increase of current amplitude, the device delivered current continuously (on-time). Once the maxi-mum amplitude was determined, the device interrupted the current delivery for 5 seconds (off-time), then delivered the current again. During this time, the peak knee extension torque generated was recorded.

Three repetitions were performed for each of the burst-duty-cycle conditions, and the mean of the 3 peak torque values for each condition, expressed as a per-centage of the MVIC torque, was calcu-lated.28,30 During testing, the investigator asked the volunteer to relax the tested limb as much as possible to avoid volun-tary influence on the electrically induced contraction. The sequence used for test-ing of the 3 burst-duty-cycle conditions (20%, 35%, and 50%) was randomized, and participants were not aware of the burst duty cycle being tested.

DiscomfortThe amount of discomfort was assessed immediately after the application of each electrical stimulation. The volunteers marked a line on a 10-cm visual analog scale, with 0 as “no discomfort” and 10

as “discomfort as bad as possible,” to in-dicate the degree of discomfort experi-enced.5 Subsequently, the distance on the line was measured with a ruler to provide a numeric value of discomfort. The mean of the 3 values corresponding to the 3 repetitions for each burst-duty-cycle con-dition was used for data analysis.

Data AnalysisDescriptive statistics were calculated for each variable of interest. The Shapiro-Wilk test was used to determine the nor-mality of the data. The Friedman test,13 followed by the Wilcoxon test, was used to compare the means of data without normal distribution (torque obtained during MVIC and percent MVIC torque). An analysis of variance with repeated measures34 was used for the analysis of normally distributed data (discomfort and current amplitudes), followed by the Tukey test. The significance level for all tests was set at an alpha of .05. Data analysis was performed using SPSS Ver-sion 11.5 (SPSS Inc, Chicago, IL).

RESULTS

One volunteer was unable to complete the tests due to acute pain during electrical stimulation,

probably from a muscle injury, and was excluded from the study. The mean SD

MVIC torque achieved by the subjects was 304 47 Nm. The maximum elec-trically induced torque with NMES was 151 65 Nm using the 20% burst duty cycle, 136 65 Nm using the 35% burst duty cycle, and 130 63 Nm using the 50% burst duty cycle.

The maximum electrically induced torques, as a percent of MVIC torque, for each of the burst-duty-cycle conditions are shown in FIGURE 2. NMES with the burst duty cycle of 20% produced a high-er maximum electrically induced torque than NMES with 35% (P = .01) and 50% (P<.01) burst duty cycles. There was no statistically significant difference in max-imum electrically induced torque gener-ated by NMES with burst duty cycles of 35% and 50% (P = .46).

No statistically significant difference was observed in sensory discomfort (P = .34) (FIGURE 3) or the maximum am-plitude of electric current tolerated (P = .62) (FIGURE 4) for the 3 burst-duty-cycle conditions.

DISCUSSION

Although medium-frequency al-ternating currents are widely used in rehabilitation and also to im-

prove athletes’ muscular performance, only a few studies have investigated the effect of different burst duty cycles on the intensity of muscle activation generated by NMES.33,37 To our knowledge, this is

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FIGURE 2. Maximum electrically induced knee extension torque with burst duty cycles of 20%, 35%, and 50%. Torque is expressed as a percentage of the torque obtained with a maximum voluntary isometric contraction. Values shown are mean SD. *Significant difference compared to 35% (P = .01) and 50% (P<.01) burst duty cycles. Abbreviations: MEIT, maximum electrically induced torque; MVIC, maximum voluntary isometric contraction.

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mFIGURE 3. Perceived discomfort during neuromuscular electrical stimulation with burst duty cycles of 20%, 35%, and 50%. Values shown are mean SD for a 10-cm visual analog scale, where 0 is “no discomfort” and 10 is “discomfort as bad as possible.” There were no statistically significant differences between the 3 conditions.

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FIGURE 4. Maximum current amplitude for neuromuscular electrical stimulation for each of the burst-duty-cycle conditions. Values shown are mean SD. There were no statistically significant differences between the 3 conditions.

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[ research report ]the first study to assess this physical pa-rameter in professional athletes. Athletes exhibit significant structural and meta-bolic differences compared to untrained individuals,41 which could lead to differ-ent electrophysiological responses.16

Burst Duty Cycle and Peak TorqueThe results of the present study demon-strate that a 20% burst duty cycle led to higher electrically induced peak torque in athletes, which is in agreement with other studies performed with nonathletic sub-jects33,37,55 but is in disagreement with the 50% burst duty cycle advocated and used by Kots.56 A possible explanation for these findings is that when bursts of medium-frequency alternating current are applied through the skin, not every cycle of alter-nating current will result in depolariza-tion of the nerve fiber. Physiologically, after each cycle of alternating current, the nerve membrane potential decreases slightly and approaches the depolarization threshold value, leading to depolarization after repeated alternating-current cycles. Therefore, the threshold voltage across the cell membrane needed for nerve excita-tion decreases as the burst duration is in-creased.36,51 Depolarization of nerve fibers according to this summation principle is known as the Gildemeister effect.

It is possible that, depending on the burst duration and current amplitude, each burst may trigger multiple action potentials as a result of summation.28,51-53 In the present study, the modulated fre-quency was 50 Hz (20-millisecond pe-riod); therefore, when using a burst duty cycle of 20%, the burst duration was 4 milliseconds. Burst duty cycles of 35% and 50% had burst durations of 7 and 10 milliseconds, respectively. Therefore, the use of medium-frequency alternating currents with bursts longer than 4 milli-seconds can decrease the nerve-fiber re-sponse due to the high number of action potentials and possibly cause synaptic fa-tigue.51,52 On the other hand, lower burst duty cycles may prevent an excessive de-polarization of nerve fiber and produce more torque, as observed in this study.

Another hypothesis to explain why the lower burst duty cycle produced a more intense contraction is that the interburst intervals cause an overall reduction in the root-mean-square of the current, allowing for a higher peak and subse-quently a stronger muscle contraction.33 Lower burst duty cycles have a longer interburst interval that could allow the use of higher current amplitude. In the present study, this cannot be confirmed because there were no differences in the amplitude used with each burst duty cycle. It is possible that burst duty cycles higher than 50% would be necessary to show differences in the current ampli-tudes delivered.

McLoda and Carmack33 analyzed the effects of different burst duty cycles to generate quadriceps femoris contrac-tion. The authors found that a burst duty cycle of 10% of alternating current (2500 Hz) produced the highest electrically in-duced torque when compared to 30%, 50%, 70%, and 90%. Similarly, Parker et al,37 using a burst-modulated alternating current with a carrier frequency of 2500 Hz, observed that the burst duty cycle of 10% produced stronger levels of quadri-ceps muscle contraction than the burst duty cycle of 90%. A previous study55 using NMES for upper-limb muscles has shown that burst duty cycles of 10% to 20% produced the highest electri-cally induced torque and that there was no significant difference between them. Burst duty cycles ranging from 20% to 50% are the ones most frequently avail-able in electrical-stimulation devices on the market. For this reason, burst duty cycles of less than 20% were not tested in this study.

In the current study, electrical stimu-lation was performed in healthy athletes who did not present muscle atrophy, in-hibition, or injury. This study did not test electrical stimulation superimposed onto voluntary muscular contraction because previous studies have not demonstrated an increase in voluntary torque or a supe-rior muscle strengthening outcome when electrical stimulation was superimposed

onto voluntary contraction in healthy subjects.10,14 Thus, using superimposed electrical stimulation would have intro-duced an extra source of error in this laboratory study.

Burst Duty Cycle and DiscomfortWard et al55 analyzed the effects of dif-ferent burst duty cycles (0.25%-100%) using different carrier frequencies (500-20 000 Hz) in untrained individuals. The cycle most frequently reported as uncom-fortable was the 100% burst duty cycle, with burst duty cycles ranging from 20% to 25% being the least often reported as uncomfortable.55 In the present study, there were no differences in discomfort among the different burst duty cycles in athletes.

In the present study, the average cur-rent amplitudes for the 3 burst-duty-cycle conditions ranged from 133 to 136 mA (current density, 2.6-2.7 mA/cm2), in contrast to 49 mA (0.82 mA/cm2) in a study using untrained men.37 The ability to tolerate very high current amplitudes could cause difficulties in the detection of differences in discomfort caused by the different burst duty cycles. Previous studies on healthy individuals who were exposed to different types of waveforms (sinusoidal, sawtooth [triangular], and square) suggested that the perception of comfort may differ among individu-als.9,17,18 Therefore, it is possible that the perception of discomfort produced dur-ing NMES may be different between athletes and untrained individuals, as athletes are able to tolerate high levels of discomfort while training.

Burst Duty Cycle and Current AmplitudeAn increase in current amplitude en-hances the evoked torque by recruiting additional motor units.22 In the current study, there was no statistical or clinically important difference among current am-plitudes used for the 3 burst-duty-cycle conditions. Thus, the highest torque pro-duced with a 20% burst duty cycle cannot be attributed to the use of higher current amplitude (intensity).

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LimitationsAnalyzing individual parameters of NMES independently can produce rele-vant clinical information on how to opti-mize NMES to achieve strength training in individuals with and without injury. The burst duty cycle is one such param-eter that has been poorly studied. The present study has limitations that must be considered when interpreting the re-sults. It is unknown whether the higher maximum electrically induced torque obtained with the 20% burst duty cycle would produce a clinically meaningful strength increase of the targeted mus-culature and a related improvement in performance. This study compared maxi-mum electrically induced torque with the use of only 3 burst duty cycles. The choice of these burst duty cycles (20%, 35%, and 50%) was based on the available burst duty cycles in most electrical-stimulation devices, making these results relevant to current clinical practice.

CONCLUSION

Medium-frequency alternating current with a 20% burst duty cycle, applied to the quadriceps,

produced a higher peak knee extension torque in professional soccer players when compared to a similar stimulus using 35% and 50% burst duty cycles. There was no significant difference in discomfort and current amplitude tol-erated among the different burst duty cycles tested. t

KEY POINTSFINDINGS: Medium-frequency alternat-ing current with a 20% burst duty cycle, compared to burst duty cycles of 35% and 50%, applied to the quadriceps, produced a higher peak knee extension torque in professional soccer players. Across the different burst duty cycles tested, there was no significant differ-ence in discomfort and current ampli-tude tolerated.IMPLICATIONS: The use of a 20% burst duty cycle may be more effective for

quadriceps strengthening in profes-sional soccer players.CAUTION: This study was performed on healthy athletes, and therefore the re-sults may not be directly applicable to individuals with injuries. It is unknown whether the noted difference would translate to additional strength or func-tional gains if NMES was used as an intervention.

REFERENCES

1. Adams GR, Harris RT, Woodard D, Dudley GA. Mapping of electrical muscle stimulation using MRI. J Appl Physiol (1985). 1993;74:532-537.

2. Arnold BL, Perrin DH, Hellwig EV. The reliability of three isokinetic knee-extension angle-specific torques. J Athl Train. 1993;28:227-229.

3. Baldi JC, Jackson RD, Moraille R, Mysiw WJ. Muscle atrophy is prevented in patients with acute spinal cord injury using functional electri-cal stimulation. Spinal Cord. 1998;36:463-469.

4. Bankov S. Medium frequency modulated im-pulse current for electric stimulation of non-de-nervated muscles. Acta Med Bulg. 1980;7:12-17.

5. Barr JO, Weissenbuehler SA, Cleary CK. Ef-fectiveness and comfort of transcutaneous electrical nerve stimulation for older per-sons with chronic pain. J Geriatr Phys Ther. 2004;27:93-99.

6. Bax L, Staes F, Verhagen A. Does neuromuscular electrical stimulation strengthen the quadriceps femoris? A systematic review of randomised controlled trials. Sports Med. 2005;35:191-212.

7. Billot M, Martin A, Paizis C, Cometti C, Babault N. Effects of an electrostimulation training program on strength, jumping, and kicking ca-pacities in soccer players. J Strength Cond Res. 2010;24:1407-1413. http://dx.doi.org/10.1519/JSC.0b013e3181d43790

8. Binder-Macleod SA, Halden EE, Jungles KA. Effects of stimulation intensity on the physiologi-cal responses of human motor units. Med Sci Sports Exerc. 1995;27:556-565.

9. Bowman BR, Baker LL. Effects of waveform parameters on comfort during transcutane-ous neuromuscular electrical stimulation. Ann Biomed Eng. 1985;13:59-74.

10. Brasileiro JS, Castro CES, Parizotto NA, Sando-val MC. Comparative study between the capacity or the torque generation and the sensorial dis-comforts produced by two forms of neuromus-cular electrical stimulation in healthy subjects. Rev Iberoam Fisioter Kinesiol. 2001;4:56-65.

11. Brocherie F, Babault N, Cometti G, Maffiuletti N, Chatard JC. Electrostimulation training effects on the physical performance of ice hockey play-ers. Med Sci Sports Exerc. 2005;37:455-460.

12. Colado JC, Garcia-Masso X, Rogers ME, Tella V,

Benavent J, Dantas EH. Effects of aquatic and dry land resistance training devices on body composition and physical capacity in postmeno-pausal women. J Hum Kinet. 2012;32:185-195. http://dx.doi.org/10.2478/v10078-012-0035-3

13. Conover WJ. Practical Nonparametric Statistics. 3rd ed. New York, NY: Wiley; 1999.

14. Currier DP, Mann R. Muscular strength develop-ment by electrical stimulation in healthy indi-viduals. Phys Ther. 1983;63:915-921.

15. Davies GJ. A Compendium of Isokinetics in Clini-cal Usage and Rehabilitation Techniques. 4th ed. Onalaska, WI: S & S Publishers; 1992.

16. Delitto A, Brown M, Strube MJ, Rose SJ, Lehman RC. Electrical stimulation of quadriceps femoris in an elite weight lifter: a single subject experi-ment. Int J Sports Med. 1989;10:187-191. http://dx.doi.org/10.1055/s-2007-1024898

17. Delitto A, Rose SJ. Comparative comfort of three waveforms used in electrically eliciting quadri-ceps femoris muscle contractions. Phys Ther. 1986;66:1704-1707.

18. Delitto A, Strube MJ, Shulman AD, Minor SD. A study of discomfort with electrical stimulation. Phys Ther. 1992;72:410-421; discussion 421-424.

19. Filipovic A, Kleinöder H, Dörmann U, Mester J. Electromyostimulation—a systematic review of the effects of different electromyostimulation methods on selected strength parameters in trained and elite athletes. J Strength Cond Res. 2012;26:2600-2614. http://dx.doi.org/10.1519/JSC.0b013e31823f2cd1

20. Gobbo M, Gaffurini P, Bissolotti L, Esposito F, Orizio C. Transcutaneous neuromuscular electrical stimulation: influence of electrode positioning and stimulus amplitude settings on muscle response. Eur J Appl Physiol. 2011;111:2451-2459. http://dx.doi.org/10.1007/s00421-011-2047-4

21. Gondin J, Cozzone PJ, Bendahan D. Is high-frequency neuromuscular electrical stimulation a suitable tool for muscle performance improve-ment in both healthy humans and athletes? Eur J Appl Physiol. 2011;111:2473-2487. http://dx.doi.org/10.1007/s00421-011-2101-2

22. Gorgey AS, Black CD, Elder CP, Dudley GA. Effects of electrical stimulation parameters on fatigue in skeletal muscle. J Orthop Sports Phys Ther. 2009;39:684-692. http://dx.doi.org/10.2519/jospt.2009.3045

23. Gorgey AS, Dudley GA. The role of pulse dura-tion and stimulation duration in maximizing the normalized torque during neuromuscular elec-trical stimulation. J Orthop Sports Phys Ther. 2008;38:508-516. http://dx.doi.org/10.2519/jospt.2008.2734

24. Gorgey AS, Mahoney E, Kendall T, Dudley GA. Ef-fects of neuromuscular electrical stimulation pa-rameters on specific tension. Eur J Appl Physiol. 2006;97:737-744. http://dx.doi.org/10.1007/s00421-006-0232-7

25. Gulick DT, Castel JC, Palermo FX, Draper DO. Effect of patterned electrical neuromuscular stimulation on vertical jump in collegiate

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926  |  december 2013  |  volume 43  |  number 12  |  journal of orthopaedic & sports physical therapy

[ research report ]

MORE INFORMATIONWWW.JOSPT.ORG@

athletes. Sports Health. 2011;3:152-157. http://dx.doi.org/10.1177/1941738110397871

26. Jaremko ME, Silbert L, Mann T. The differential ability of athletes and non-athletes to cope with two types of pain: a radical behavioral model. Psychol Record. 1981;31:265-275.

27. Johnson MH, Stewart J, Humphries SA, Chamove AS. Marathon runners’ reaction to potassium iontophoretic experimental pain: pain tolerance, pain threshold, coping and self-efficacy. Eur J Pain. 2012;16:767-774. http://dx.doi.org/10.1002/j.1532-2149.2011.00059.x

28. Laufer Y, Ries JD, Leininger PM, Alon G. Quad-riceps femoris muscle torques and fatigue generated by neuromuscular electrical stimula-tion with three different waveforms. Phys Ther. 2001;81:1307-1316.

29. Lewek M, Stevens J, Snyder-Mackler L. The use of electrical stimulation to increase quadriceps femoris muscle force in an elderly patient fol-lowing a total knee arthroplasty. Phys Ther. 2001;81:1565-1571.

30. Lyons CL, Robb JB, Irrgang JJ, Fitzgerald GK. Differences in quadriceps femoris muscle torque when using a clinical electrical stimulator versus a portable electrical stimulator. Phys Ther. 2005;85:44-51.

31. Maffiuletti NA, Cometti G, Amiridis IG, Martin A, Pousson M, Chatard JC. The effects of electro-myostimulation training and basketball practice on muscle strength and jumping ability. Int J Sports Med. 2000;21:437-443. http://dx.doi.org/10.1055/s-2000-3837

32. Manning EL, Fillingim RB. The influence of athletic status and gender on experimental pain responses. J Pain. 2002;3:421-428.

33. McLoda TA, Carmack JA. Optimal burst duration during a facilitated quadriceps femoris contrac-tion. J Athl Train. 2000;35:145-150.

34. Montgomery DC. Design and Analysis of Experi-ments. 5th ed. New York, NY: Wiley; 2001.

35. Moreno-Aranda J, Seireg A. Investigation of over-the-skin electrical stimulation parameters for different normal muscles and subjects. J Biomech. 1981;14:587-593.

36. Palmer ST, Martin DJ, Steedman WM, Ravey J. Alteration of interferential current and transcu-taneous electrical nerve stimulation frequency: effects on nerve excitation. Arch Phys Med Reha-bil. 1999;80:1065-1071.

37. Parker MG, Broughton AJ, Larsen BR, Dinius JW, Cimbura MJ, Davis M. Electrically induced contraction levels of the quadriceps femoris muscles in healthy men: the effects of three pat-

terns of burst-modulated alternating current and volitional muscle fatigue. Am J Phys Med Reha-bil. 2011;90:999-1011. http://dx.doi.org/10.1097/PHM.0b013e318238a2cf

38. Parker MG, Keller L, Evenson J. Torque respons-es in human quadriceps to burst-modulated alternating current at 3 carrier frequencies. J Orthop Sports Phys Ther. 2005;35:239-245. http://dx.doi.org/10.2519/jospt.2005.35.4.239

39. Pichon F, Chatard JC, Martin A, Cometti G. Elec-trical stimulation and swimming performance. Med Sci Sports Exerc. 1995;27:1671-1676.

40. Piva SR, Goodnite EA, Azuma K, et al. Neuro-muscular electrical stimulation and volitional exercise for individuals with rheumatoid arthri-tis: a multiple-patient case report. Phys Ther. 2007;87:1064-1077. http://dx.doi.org/10.2522/ptj.20060123

41. Ribeiro SR, Tierra-Criollo CJ, Martins RA. Effects of different strengths in the judo fights, muscular electrical activity and biomechanical param-eters in elite athletes. Rev Bras Med Esporte. 2006;12:23e-28e. http://dx.doi.org/10.1590/S1517-86922006000100006

42. Robertson VJ, Ward AR. Vastus medialis electrical stimulation to improve lower ex-tremity function following a lateral patellar retinacular release. J Orthop Sports Phys Ther. 2002;32:437-443; discussion 443-446. http://dx.doi.org/10.2519/jospt.2002.32.9.437

43. Rooney JG, Currier DP, Nitz AJ. Effect of varia-tion in the burst and carrier frequency modes of neuromuscular electrical stimulation on pain perception of healthy subjects. Phys Ther. 1992;72:800-806; discussion 807-809.

44. Selkowitz DM, Rossman EG, Fitzpatrick S. Effect of burst-modulated alternating current carrier frequency on current amplitude required to pro-duce maximally tolerated electrically stimulated quadriceps femoris knee extension torque. Am J Phys Med Rehabil. 2009;88:973-978. http://dx.doi.org/10.1097/PHM.0b013e3181c1eda5

45. Stevens JE, Mizner RL, Snyder-Mackler L. Neuromuscular electrical stimulation for quadriceps muscle strengthening after bilateral total knee arthroplasty: a case series. J Orthop Sports Phys Ther. 2004;34:21-29. http://dx.doi.org/10.2519/jospt.2004.34.1.21

46. Stevens-Lapsley JE, Balter JE, Wolfe P, Eckhoff DG, Kohrt WM. Early neuromuscular electri-cal stimulation to improve quadriceps muscle strength after total knee arthroplasty: a random-ized controlled trial. Phys Ther. 2012;92:210-226. http://dx.doi.org/10.2522/ptj.20110124

47. Stevens-Lapsley JE, Balter JE, Wolfe P, et al. Relationship between intensity of quadriceps muscle neuromuscular electrical stimulation and strength recovery after total knee arthro-plasty. Phys Ther. 2012;92:1187-1196. http://dx.doi.org/10.2522/ptj.20110479

48. Tesarz J, Schuster AK, Hartmann M, Gerhardt A, Eich W. Pain perception in athletes compared to normally active controls: a systematic review with meta-analysis. Pain. 2012;153:1253-1262. http://dx.doi.org/10.1016/j.pain.2012.03.005

49. Vanderthommen M, Triffaux M, Demoulin C, Crielaard JM, Croisier JL. Alteration of muscle function after electrical stimulation bout of knee extensors and flexors. J Sports Sci Med. 2012;11:592-599.

50. Vaz MA, Baroni BM, Geremia JM, et al. Neu-romuscular electrical stimulation (NMES) reduces structural and functional losses of quadriceps muscle and improves health status in patients with knee osteoarthritis. J Orthop Res. 2013;31:511-516. http://dx.doi.org/10.1002/jor.22264

51. Ward AR. Electrical stimulation using kilohertz-frequency alternating current. Phys Ther. 2009;89:181-190. http://dx.doi.org/10.2522/ptj.20080060

52. Ward AR, Oliver WG. Comparison of the hypoal-gesic efficacy of low-frequency and burst-mod-ulated kilohertz frequency currents. Phys Ther. 2007;87:1056-1063. http://dx.doi.org/10.2522/ptj.20060203

53. Ward AR, Robertson VJ. The variation in fatigue rate with frequency using kHz fre-quency alternating current. Med Eng Phys. 2000;22:637-646.

54. Ward AR, Robertson VJ. Variation in motor threshold with frequency using kHz fre-quency alternating current. Muscle Nerve. 2001;24:1303-1311.

55. Ward AR, Robertson VJ, Ioannou H. The ef-fect of duty cycle and frequency on muscle torque production using kilohertz frequency range alternating current. Med Eng Phys. 2004;26:569-579. http://dx.doi.org/10.1016/j.medengphy.2004.04.007

56. Ward AR, Shkuratova N. Russian electrical stimulation: the early experiments. Phys Ther. 2002;82:1019-1030.

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